FRIDAY, March 18 (HealthDay News) -- Orthodontic retainers, removable devices used to keep teeth straight, can develop a build-up of potentially harmful microbes if they aren't properly cleaned, finds a new study.

Researchers at the UCL Eastman Dental Institute checked the retainers of a group of study participants and found that about 67 percent had a type of yeast called Candida that can cause fungal infections, while 50 percent had Staphylococcus bacteria, including methicillin-resistant Staphylococcus aureus (MRSA).

Candida and Staphylococcus rarely cause problems in healthy people but can pose a serious threat to people with weakened immune systems, noted study author Jonathan Pratten and colleagues.

Further investigation revealed that the bacteria on retainers live in biofilms, which are communities of bacteria living together in a layer of slime. These biofilms can be difficult to remove and tend to be highly resistant to antimicrobial agents.

The findings, published online March 14 in the journal Letters in Applied Microbiology, suggest the need for improved cleaning products for orthodontic retainers.

The researchers said anyone handling a retainer should wash their hands before and after use. Careful tooth brushing and the use of mouthwash may also help keep the retainer clean.

"With the growing awareness the public has of hospital-acquired infections, it is important to be aware of other potential 'hidden reservoirs' of harmful bacteria which could be introduced to environments where we know they can cause problems," Pratten stated in a journal news release.